import java.util.Scanner;

/**
 * Created with IntelliJ IDEA.
 * Description:
 * User: 666
 * Date: 2023-04-14
 * Time: 14:32
 */
public class test0 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String str = scanner.next();
        int[] arr = new int[26];
        for (int i = 0; i < str.length(); i++) {
            arr[str.charAt(i) - 'a']++;
        }
        for (int i = 0; i < str.length(); i++) {
            if ( arr[str.charAt(i) - 'a']==1) {
                System.out.println(str.charAt(i));
                break;
            }
        }

        }
    }
